1. Field of the Invention
The present invention relates to a gyroscope, more particularly to a direction-switched mode-locked laser gyroscope.
2. Description of the Prior Art
Lasers have been used only as light sources of various optical systems. However, sensors using the characteristic change of lasers depending on external physical parameters were recently suggested. Especially, with the development of optical fiber lasers, fiber laser sensors using the optical fiber laser are developed. The fiber laser sensor has many advantages of simple system configuration, simple signal-processing and the like.
Two types of optical rotation sensors have been developed over the past decade: the ring laser gyroscope and the interferometric fiber-optic gyroscope.
FIG. 1 schematically shows a ring laser gyroscope of the prior art. Referring to FIG. 1, a laser gain medium 104 is disposed in a laser resonator composed of two reflecting mirrors 101, 102 and a partially reflecting mirror 103. The ring laser gyroscope of the above configuration is disclosed in the following reference : W. W. Chow et al. "The ring laser gyro", Rev. of Mod. Phys., Vol. 57, No. 1, P61, January 1985. Therefore, detailed explanation of the ring laser gyroscope will be omitted. Briefly explaining the operation of the gyroscope, laser gain medium 104 emits light by the supply of external energy, that is, optical pumping. The light is amplified in the resonator. Some of the amplified light, that is laser beam, passes through the partially reflecting mirror. There are two modes in the resonator : the first one 109 that is rotating clockwise, and the second one 110 that is rotating counterclockwise. The output laser beams emitting from partially reflecting mirror 103 interfere with each other after being combined by a beam splitter 107. The interference signals are then detected by a photo detector 108. If the resonator rotates, two modes 109 and 110 experience different resonator lengths, respectively. Hence they have different frequencies and the rotation rate can be measured by detecting the beat frequency at photo detector 108. The ring laser gyroscope using He-Ne gas gain medium is already commercialized, but it has relatively short lifetime and shows low durability to mechanical shock.
FIG. 2 schematically shows the configuration of an interferometric fiber-optic gyroscope different from the ring laser gyroscope. The feature of its configuration is described in the following reference: R. A. Bergh, H. J. Shaw, "An overview of Fiber-Optic Gyroscope", J. of Lightwave Technology, Vol. LT-2, P91, 1984.
Referring to FIG. 2, a light beam from a light source 201 is split by 50:50 at a 3-dB directional coupler 203 and the separated beams propagate in opposite directions around a Sagnac loop 206. The two beams are recombined at directional coupler 203. If the two beams experience different phases whose difference is .phi., the light intensity detected by photo detector 204 after passing through directional coupler 202 is proportional to 1+cos(.phi.). If this system rotates, the rotation rate can be obtained by measuring the intensity of output light beams since .phi. is proportional to the rotation rate. In general, the Sagnac interferometer employs expensive polarization maintaining optical fiber to eliminate error factors, which results in high system cost. Moreover, high-performance polarizers and complex signal-processing to read phase difference also make it difficult to lower the cost.
Recently a mode-locked fiber laser gyroscope was suggested. FIG. 3 shows the basic configuration of the mode-locked fiber laser gyroscope. The feature of this gyroscope is described in the following reference: "Mode-Locked Fiber Laser Gyroscope", Opt. Lett., Vol. 13, No. 4, P320, Feb. 15, 1993.
Referring to FIG. 3, one end of the laser resonator is composed of a reflector 301 and the other end of the resonator is composed of a Sagnac interferometer 303. Laser light can be generated by optically pumping the gain medium 302. The power of the output light 307 from Sagnac interferometer 303 has a value of (1+cos.phi..sub.nr)/2 multiplied by the power of the input light 308. Here, .phi..sub.nr represents a nonreciprocal phase. If .phi..sub.nr is zero, the output power from the Sagnac interferometer is equal to the input light, which explains the fact the interferometer 303 is also called as Sagnac loop mirror. The nonreciprocal phase .phi..sub.nr can be modulated to satisfy .phi..sub.nr =.phi..sub.M sin.omega.t by phase modulator 304 inside the Sagnac loop mirror, which results in the modulated reflectivity of the loop mirror.
If the period of the modulation is adjusted to be same with the round trip time, mode locking occurs and a train of laser pulses is generated instead of continuous wave laser light. The reflectivity-modulation of a stationary loop mirror would give two pulses for one period of phase modulation as shown in FIG. 4B. If the laser rotates, .phi..sub.nr is changed to .phi..sub.nr =.phi..sub.R +.phi..sub.M sin.omega.t, where the nonreciprocal phase .phi..sub.R is induced by the rotation, as shown in FIG. 4A. In this case, the spacing between the two pulses is altered since the reflectivity varies as shown in FIG. 4C. Therefore, the rotation rate can be obtained by measuring the spacing between the two pulses in the mode-locked fiber-optic gyroscope shown in FIG. 3.
However, the measurement of the spacing between pulses in the signal processing is much more difficult compared with the frequency measurement in the conventional ring laser gyroscope or the gyroscope according to the present invention.